"I am the only way he will ever decide to try with God"
That right there is the central problem. Guess what? God doesn't 'need' you to get to him. And the fact that you are so emotionally entangled with this guy is probably more of a hindrance because you are pushing too hard.
And really, the fact that you told him that you won't date someone not Christian... that failed. Because you already were dating him. And you went back with him After saying that.
So many wrong attitudes and mindsets and beliefs and choices coming from you, yet you seem to think you are the 'only way' this guy can find God. Quite frankly you've been rather wishy washy about things, and that reflects poorly on God. You've already shown him you'll stay with him and disregard your standards to stay with him. What does that say about your faith? What does that teach him about God and being a Christian? Say one thing and do another? What does that say about your personal Character? That you can be swayed?
How about telling him how bad of an idea missionary dating is, then turning around and doing the very thing you Told Him was Bad?
Bottom line is you have done quite poor job representing God, and not showing a strength of character. You contradict yourself at every turn and the only real stance you've taken isn't about God, but about staying with this guy.
I'm not trying to be mean. I'm just giving you the facts. I understand when you're in love it's easy to be blind to our own choices. It's easy to ignore actions we normally would not allow ourselves to do. I've been guilty of it myself numerous times. And that's why it's so easy for me to spot you doing it. But right now your emphasis is on the 'guy' and not truly on God.
So my advice to you is to stop doing the opposite of what you keep telling him. Gain your character back. Recognize that you are not God, nor above Him, and that you are Not the 'only' person that can sway this guys mind. That's not your job. And if you are dating, or want to date, you are usually the last person that should be trying to be a witness because, deep down, your motives are selfish, not Godly. Not to mention the chances of him going along with things to keep you is greatly increased as well, making him never saved, only pretending. And eventually the truth will come out. I hope this helps. Take care.